1. Field of the Invention
The present invention relates in general to extruder and injection molding screws of the type used for the melting and movement of polymeric material within a plasticizing cylinder from an inlet feed opening to the discharge opening at the end of the cylinder. In particular, the present invention relates to such an extrusion or injection screw having an improved mixing section which achieves dispersive and distributive mixing of the polymer without excessive shear while operating at a high RPM or production rate.
2. Description of the Prior Art
Extruder and injection screw design technology has been known for many years. The typical configuration includes a feed section, a transition section and a meter section arranged along the length of the screw, which is rotatably housed within a cylinder. The polymer material (or resin) is fed into the cylinder at the feed section of the screw. A helical land (or flight) on the screw serves to convey the resin forward in the cylinder upon rotation of the screw.
The resin moves within a helical channel defined by the screw flights, the screw shaft (or root) and the internal wall of the cylinder. As the resin enters the transition section, the root becomes gradually larger in diameter and the screw channel, bounded by the continuing flight, therefore becomes gradually more shallow. The cylinder is externally heated and the resin is compressed, conveyed and initially melted as it is moved forward in the progressively smaller screw channels of the transition section. The resin then moves into the metering section where final melting of any previously unmelted particles is completed.
The metering section typically has a shallow screw channel but a constant root diameter which, in combination with the continuing helical screw flight and screw rotation, moves the melted resin forward toward the discharge opening. The resin is forced through the discharge opening either by the continued rotation of the extruder screw, or by an injection stroke forward of an injection screw.
Some screw designs are altered to provide for a secondary (or barrier) flight which is located in the transition section, and which is undercut or spaced a short distance from the cylinder wall, allowing a separation of the melt from unmelted solid resin by the flow of melted resin over the undercut barrier flight as the material is conveyed forward in the plasticizing process. Barrier screws are designed to provide an efficient melting of the polymer achieving a high melt quality while operating at a slower RPM. Such barrier screw designs are now common, and the Willert screw disclosed in U.S. Pat. No. 4,330,214 is a notable example. While barrier screws have a particular application, they cannot be operated at the relatively high speeds necessary for many commercial operations without overworking the polymer. Many applications require that the plasticizing equipment be operated at relatively high speeds to obtain the desired economic efficiencies. Faster production rates may be used but may result in the barrier screw churning the material and creating stresses in the parts formed as end products from the material, as well as parts not likely to retain the required dimensional consistency and accuracy, due to overworking the polymer. The additional stresses must be relieved by increasing the "clamp time", or the time the part remains in the mold, to avoid distortion of the stressed material upon cooling. In addition, the Willert reference discloses a screw with no inlet channel, and the helix angle of the barrier and wiping lands are both the same angle. Barrier screws accomplish a limited mixing of the melt.
For the foregoing and other reasons, it is often considered desirable to utilize metering screws in a plasticizer and to also provide a relatively short mixing section in the metering section to provide improved mixing of the melt and to help insure a complete melting of all resin solids. A mixing section for a metering screw is required for disbursing solids such as coloring throughout the extrudate, and is generally located at or near the end of the metering section of a standard metering screw or a barrier screw. Mixing section designs are also common in the industry, of which U.S. Pat. No. 4,752,136 to Colby, U.S. Pat. No. 3,941,535 to Street, and U.S. Pat. No. 3,788,614 to Gregory are notable examples. An early design by LeRoy, U.S. Pat. No. 3,486,192, originally assigned to and known as the Union Carbide mixer, is still employed in the industry.
It is generally agreed that all or substantially all of the resin must be melted prior to entering the mixing section of a screw to enable satisfactory mixing action. This melting process may be accomplished either by a properly designed three-zone meter screw configuration or a barrier screw configuration, which precedes the mixing section. Much prior art is known relative to the design of these configurations, some of which has been referenced previously. It is well-known that neither standard meter screw nor barrier screw configurations achieve optimum mixing of the melt without incorporating a downstream mixing section. Willert purports to achieve this final mixing or blending of the melt through reversing the action of the screw by changing the barrier land to a wiping land and the wiping land to a barrier land at the forward end of the barrier zone. Others simply add a mixing section in the meter zone of their screws such as Colby, Street, or Gregory. The present invention provides an improved mixing section of this type.
Colby's mixing section design embodies a helical valley in the metering section of the screw divided into side-by-side relatively shallow and relatively deep levels extending along the valley in side-by-side helical paths. Each path has a different pitch than the pitch of the adjacent helical flight so that the helical flight periodically interrupts the helical paths. During operation, the level differential induces a continuous tumbling and mixing action upon the molten resin in the mixing section.
U.S. Pat. No. 3,941,535 to Street discloses a mixing section having a plurality of helical channels Without dead ends. The lands or protuberances extending between the channels are of equal height, but all lands contain notches therein creating passageways for separating and recombining the melt to effect high volume mixing. The lands are undercut from a normal screw flight height, and are therefore spaced from the cylinder bore to give intensive high shear treatment to the melt in the space therebetween. However, the protuberances are all undercut to a barrier land height and the mixer contains no wiping lands to assure minimum screw recovery time in an injection screw or pumping effectiveness in an extruder screw. Moreover, the design of the notches does not facilitate rapid color change.
U.S. Pat. No. 3,788,614 to Gregory discloses a mixing section having inlet and outlet grooves or channels arranged in a helical manner. These inlet and outlet channels are of equal width, with the inlet channels having dead ends which cause the melt to flow over the adjacent land into the outlet channel or over the closed end of the inlet channel to exit the mixing section. The mixing section of Gregory, however, is of uniform diameter (i.e., the lands all have the same height) with the channels formed therein, providing dispersive mixing and temperature uniformity of readily degradable polymers or mixtures containing such. However, Gregory does not provide for wiping lands which are needed for pumping ability and enhancement of the heat transfer from the heating elements exterior of the plasticizer to the plastic material in the apparatus. In addition, the closed ends of the inlet channels form dead spots which are not conducive to producing isothermal melt quality and they impede rapid color change.
LeRoy's design is similar to Gregory's, except that the inlet and outlet channels are arranged longitudinally in the mixing section and the lands defining the channels alternate in height with one being a wiping land with substantially the same clearance from the cylinder wall as the screw flight. The wiping lands force the molten polymer over barrier lands into the outlet channels to exit the mixing section. By this action, the LeRoy design purports to eliminate unmelted solids and provide a mixing action for the melt.
U.S. Pat. No. 3,652,064 to Lehnen, et al. embodies two mixing sections in an extruder screw designed to process highly viscous materials, particularly synthetic or natural rubber stocks. The first mixing section has a countercurrent land with interruptions therein for partially backfeeding or kneading the material to prepare it for subsequent melting and conveying. The second section has two threads or lands of the same height, one of which has radial grooves or gaps therein, to divide the material into partial streams flowing backwards but having further travel under a substantially steady forward feed to compact and intensively mix the material. The two lands have a different pitch (and width) forcing the flow of the material through the gaps into the screw channel ensuring good compacting and homogenization of the material. The two lands taper off free at the start and the end so that the channels they define have free access to the adjacent channels. All of the typical dimensions of the apparatus relate to processing synthetic butyl rubber or natural rubber. The backfeeding or kneading of material in the first mixing section is desirable for preparing rubber for subsequent melting and conveying but creates undesirable excessive shear in thermoplastic material which can result in degradation. The second mixing section also causes a backward flow and shearing of the material creating a significant pressure drop and restricts the use of higher RPM's in processing thermoplastics.